This invention relates to the casting of steel strip.
It is known to cast metal strip by continuous casting in a twin roll caster. In this technique molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term "nip" is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
Although twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals. One particular problem has been the achievement of sufficiently rapid and even cooling of metal over the casting surfaces of the rolls. In particular it has proved difficult to obtain sufficiently high cooling rates for solidification onto casting rolls with smooth casting surfaces and it has therefore been proposed to use rolls having casting surfaces which are deliberately textured by a regular pattern of projections and depressions to enhance heat transfer and so increase the heat flux achieved at the casting surfaces during solidification.
Although various forms of surface texture have been proposed, we have determined that the most successful texture in terms of achieving increased heat flux during solidification is one formed by a series of parallel groove and ridge formations. More specifically, in a twin roll caster the casting surfaces of the casting rolls may be textured by the provision of circumferentially extending groove and ridge formations of essentially constant depth and pitch. The reasons for the enhanced heat flux obtained with casting surfaces of this formation are fully explained in our Australian Patent Application NO 50775/96 entitled CASTING STEEL STRIP. This application further describes how the texture can be optimised for casting of steel in order to achieve both high heat flux values and a fine microstructure in the as cast steel strip. Essentially when casting steel strip, the depth of the texture from ridge peak to groove root should be in the range 5 microns to 50 microns and the pitch of the texture should be in the range 100 to 250 microns for best results. For optimum results it is preferred that the depth of the texture be in the range 15 to 25 microns and that the pitch be between 150 and 200 microns.
Although the use of textured casting surfaces enables sufficiently high heat flux values to be obtained on solidification to enable satisfactory casting of steel strip the resulting strip can suffer from surface defects caused by deposition of solid oxides on the casting surfaces during initial solidification within the casting pool, the solid sides being present as de-oxidation products in the molten steel. Ferrous metals are particularly prone to deposit solid inclusions by producing oxides in solid form at the casting temperature. The deposition of Al.sub.2 O.sub.3 is a particular problem. Such deposition can lead to intermittent contact between the textured casting surfaces and the melt at the initial point of contact between the melt and the casting surface in the casting pool (ie the meniscus region) which results in a transverse surface depression in the resulting cast strip, the defect being known as "chatter". We have now determined that it is possible to avoid surface defects caused by deposition of solid oxides (de-oxidation products) by ensuring that each casting surface is covered by a thin layer of material a major proportion of which layer remains liquid as the steel is cooled below its liquidus temperature in the formation of the solidified shell on the casting surface. The interposition of such a substantially liquid layer between the casting surface and the cooling steel in the casting pool can result in substantial under-cooling of the steel below its liquidus temperature before the metal solidification is complete because it suppresses the availability of discrete nucleation sites. Because the layer is substantially liquid during the metal solidification, it suppresses the formation of defects in the solidifying metal surface due to early deposition of solid oxides on the casting surfaces, the term "metal solidification" being used herein to refer to the extended solidification period when the molten steel is cooled below its liquidus temperature.